Nicotine addiction is a manifold process that involves dysregulated executive control, reward processing and negative affect. Preclinical models reveal that self-administration of nicotine produces an imbalance in corticostriatal glutamate transmission that mediates cue-induced reinstatement to nicotine seeking. Prior research suggests that N- acetylcysteine (NAC), a modified cysteine chain that regulates glial glutamate and normalizes cocaine-induced synaptic plasticity, may attenuate perceived smoking reward and smoking behavior. Crucially however, our preclinical data suggests NAC may be most effective under conditions of abstinence as a relapse prevention aid. Thus, pairing a smoking cessation medication (Varenicline: VRN) that promotes abstinence, with a relapse prevention aid (NAC), may produce greater cessation outcomes than either treatment alone. The overarching goal of this proposal is to use clinical and preclinical neuroscience to investigate limbic-striatal and corticostriatal circuits involved in nicotine addiction and determine the value of VRN+NAC for normalizing circuitry function and treating nicotine addiction. Smokers interested in quitting will be randomized to one of four placebo (PBO) controlled conditions to examine the individual and combined effects of VRN and NAC over 4-weeks. Following 1-week of treatment, participants will be contingently reinforced for 3 days of smoking abstinence and undergo an fMRI scan. Participants will be followed over the next 3-weeks of treatment and clinically relevant behaviors will be assessed. Relations between fMRI measures and clinical variable will be explored. In our analog preclinical studies that use the same treatment groups as the clinical studies, we will employ an optogenetic strategy to examine the efficacy of VRN+NAC at inhibiting synaptic plasticity in prelimbic (PL) and basolateral amygdala (BLA) inputs to the nucleus accumbens produced during cue-induced nicotine seeking.